In the known vacuum heat treating furnaces, the metallic workload is heat treated in a hot zone and subsequently cooled with a cooling gas. The cooling gas is injected into the hot zone through one or more nozzles that penetrate through the hot zone wall. The nozzles have unobstructed channels that reduce inert gas partial pressure and allow heat to escape from the hot zone during the heating portion of a heat treatment cycle. The gas pressure and heat loss result in poor temperature uniformity around the workpiece. In order to overcome this problem, some vacuum heat treating furnaces include valves or other hardware connected to the cooling gas nozzles on the inside of the hot zone. The valves allow cooling gas to enter into the hot zone through the nozzles, but limit the escape of gas partial pressure and heat through the gas injection nozzles during the heating cycle.
Valves installed in the interior of the hot zone are subject to breaking and wear in a short period of time, because many have moving parts that cannot withstand repeated exposure to the high temperatures in the hot zone. In addition, many of the known valves are formed from materials that cannot withstand such high temperatures. Failure of these devices can create significant down time, because the furnace and hot zone must be opened to access the broken or worn valve. Also, when the valves are arrayed radially about the interior of the hot zone, special measures must be implemented to maintain some of the valves in a closed position because the force of gravity tends to open them. It can be seen that the devices presently used to limit the loss of pressure and temperature from hot zones have limitations that cause them to fall short of the needs of those who operate such furnaces.